Screw-type closures molded of resilient plastic materials such as polyethylene or polypropylene have been widely used for sealing metal, glass, and plastic containers holding a variety of products. However, closures of this type have been of limited practicability for liquids under pressure, such as carbonated beverages. This limitation is due not only to the permeability of the pliable materials used, and to the unavoidable inaccuracies of the mating parts, both of which become more critical with increasing pressure, but may also be the unwanted side-effect of some efforts to improve pressure retention. For example, screwing the closure more tightly onto the container may deform the thread so much that it will be difficult for the user to unscrew. In addition it may cause distortion of the closure, due to unbalanced loading of the screw thread, to the point where leakage is actually increased.
Separate sealing materials have sometimes been added to a basic closure to improve these conditions, but they add substantially to the cost. Also, many types of integrally-molded sealing lips have been designed for incorporation into the body of the closure, with the aim of reducing leakage. Some of these seal designs are arranged to bear on the inside or the outside of the rim of the container opening, in which case the amount of sealing pressure which can consistently be applied to them is restricted by the tensile strength and resilience of the material used, and by the unavoidable dimensional variations of the mating parts. Others are arranged to bear on the top surface or edges of the container rim. In this approach, the characteristics of the material and the dimensional variations of the parts are less critical, owing to the accommodative ability of the screw thread, but the sealing action becomes more sensitive to any distortion or cocking of the closure. It is this condition which the present invention is principally designed to improve.
In the conventional construction there is an inherent tendency toward distortion and cocking, due to the fact that both the closure and container threads always have the same axial pitch spacing. Such exact matching may be correct practice in ordinary situations, but it results in uneven stresses when one of the mating parts is substantially more pliable than the other. In the case of resilient plastic closures for pressurized applications, the thread is formed on a body of relatively pliable material, whereas the mating container thread is much more rigid. As a result, the point of thread contact nearest the seal elements receives a substantially greater proportion of the sealing load than the one farthest away, the load on which is weakened by stretching of the additional plastic material above it. Between these two limits, there is a gradual reduction of the load assumed by each portion of the thread. The net result is that the portion of the seal directly above the topmost contact point is compressed more heavily than desirable, while other portions of the seal are compressed more lightly. This results in a tendency for the closure to tilt. It may also lead to jamming of the thread at the highest point of contact during installation, and in extreme cases to distortion or rupture of the plastic wall. These conditions in turn may lead to difficulty in manually removing the closure from the container.